1. Technical Field
The present invention relates to a transfer device configured to transfer an image formed on an image carrier to a recording medium and an image forming apparatus provided with the device.
2. Related Art
Image forming apparatuses which develop and form electrostatic latent images using a high-viscosity liquid developer including toner formed of solid component dispersed in carrier liquid, are currently known in the art. One example of one of these image forming apparatuses is in FIG. 1 of Japanese Patent Document No. JP-A-2009-14808.
In the '808 image forming apparatus, four image forming stations configured to form images of different colors are provided and are used to form toner images which are superimposed on an intermediate transfer belt, thereby forming a multi-color image on the intermediate transfer belt. The intermediate transfer belt is extended between a pair of drive roller and a driven roller which are disposed apart from each other. The intermediate transfer belt is rotated in a predetermined direction while carrying the color image by driving the drive roller using a belt drive motor. A transfer device often used in a so-called dry image forming apparatus is provided so as to oppose the drive roller, and the color image is secondarily transferred to the recording medium, which may comprise a variety of different types of media, including as sheets of paper, films, cloths, and so on. In other words, a secondary transfer roller covered with an elastic member such as rubber over an outer circumference thereof is disposed at a secondary transfer position so as to oppose the drive roller with the intermediate transfer belt being disposed between the secondary transfer roller and the drive roller, thereby forming a nipping area with respect to the intermediate transfer belt.
Then, by driving the drive roller using the belt drive motor, the intermediate transfer belt is rotated by the intermediate transfer belt and the secondary transfer roller is driven and rotated, causing the recording medium to be pinched and transported at the nipping area. During the transportation in the nipping point, the color image formed on the intermediate transfer belt is secondarily transferred to the recording medium.
Additional examples of apparatuses known in the art are illustrated in FIG. 1 of JP-A-2009-14808 and FIG. 1 of JP-A-2000-238400.
Since the apparatus disclosed in JP-A-2009-14808 employs a wet system using the liquid developer, difficulties may arise when the recording medium which has passed through the nipping point and is being transported toward the image forming station because the recording medium may become stuck to the surface of the intermediate transfer belt, resulting in a paper jam. In order to solve this problem, usage of a transfer device such as those currently employed in a stencil printing apparatus has been proposed.
More particularly, in the stencil printing apparatus, such as the apparatus described in JP-A-2000-238400, an impression cylinder having a sheet clamper for holding a leading end of the sheet is provided as the transfer device. In other words, the impression cylinder (corresponding to “transfer roller” of the invention) is registered at a fixed position with respect to a cylinder on which a pressed master is wound. Then, the impression cylinder is rotated by a drive force provided from a driving portion such as a motor while pinching the leading end of the sheet by the sheet clamper, so that ink printing may be performed.
Thus, using the sheet clamper, the printed sheet is not stuck to the cylinder and is reliably separated from the cylinder on the downstream side of the pressed position even though high-viscosity ink is used. Because this system works reliably for an stencil printing apparatus, attempts have been made to configure the secondary transfer roller in the same manner as the impression cylinder, that is, to provide the secondary transfer roller with a gripping portion which may be used to grip the recording medium on the secondary transfer roller. In other words, by driving the secondary transfer roller while gripping the leading end (non-image portion) of the recording medium with the gripping portion, the recording medium having passed through the nipping point is separated desirably from the intermediate transfer belt, and paper jams can be effectively prevented.
However, when diverting the transfer device disclosed in JP-A-2000-238400 to the image forming apparatus, two points are needed to be taken into consideration and, consequently, load variations might become a serious problem. The first point to be considered is a contact system of the secondary transfer roller with respect to the intermediate transfer belt. In other words, in the image forming apparatus of this type, the transfer process is performed by pressing the secondary transfer roller toward the intermediate transfer belt such that the secondary transfer roller is in contact with the intermediate transfer belt via the recording medium with a constant load being applied. It is important that the load applied remain constant in order to transfer the images desirably to the recording media having various thicknesses. Therefore, it is necessary that a constant load system is used when diverting the transfer device disclosed in JP-A-2000-238400.
The second point is that it is necessary to provide a concaved portion on the outer periphery of the secondary transfer roller and dispose the gripping portion in the concaved portion. In other words, while the concaved portion opposes the intermediate transfer belt, the recording medium is needed to be gripped in a state in which the outer circumference of the secondary transfer roller is kept separate from the surface of the intermediate transfer belt.
Then, when the concaved portion does not oppose the intermediate transfer belt, that is, when the outer circumference of the secondary transfer roller opposes the surface of the intermediate transfer belt and forms the nipping point, the toner image carried on the intermediate transfer belt is transferred to the recording medium.
In this manner, although a period in which the outer periphery of the secondary transfer roller opposes the intermediate transfer belt (contact period) and a period in which they are separated (separation period) are mixed while the secondary transfer roller makes one turn, the secondary transfer roller is pressed toward the intermediate transfer belt. Therefore, the load applied on the intermediate transfer belt may vary significantly when the contact period is switched to the separation period or vice versa. In particular, in the stencil printing apparatus disclosed in JP-A-2000-238400, since the printing is performed using the master which has been pressed in advance, the variation of the load does not cause a serious problem. In the image forming apparatus, however, lowering of the image quality due to the load variations may occur. In other words, in the image forming apparatus, image forming processes such as formation of a latent image, development, and a primary transfer of the toner image is needed to be performed in parallel with the rotation of the secondary transfer roller.
As a consequence of the load variations which may occur during the image forming processes, problems such as fluctuations in the latent image at the latent image forming position (exposing portion) or fluctuations in the primarily transferred toner image. These fluctuations may lower the image quality.